1. Field of the Invention
This invention relates to a two-colored molded product of polyurethanes made by a reactive injection molding (RIM), which has a surface portion and an inner portion differing from each other in color and properties, and a process for manufacturing the same.
2. Description of the Related Art
A conventional RIM process for manufacturing a molded product of polyurethanes usually includes such steps as will be stated below with reference to FIGS. 8 to 10:
(1) A mold 100 is opened, and the wall 101 of its cavity is coated with a mold release agent 102 which is applied by e.g. spraying, as shown in FIG. 8.
(2) An in-mold coating material 103 is applied by e.g. spraying along the cavity wall 101 so as to lie on the mold release agent 102. The in-mold coating material 103 is a mixture of a polyurethane material of high light resistance and a coloring agent.
(3) An insert 104, if any, is set in the cavity, and the mold 100 is closed.
(4) A RIM polyurethane material 105 is injected into the cavity and allowed to undergo reaction and curing, as shown in FIG. 9.
(5) The mold 100 is opened, and a molded product of polyurethanes is removed from the mold. The molded product 106 has a surface portion formed from the in-mold coating material 103 and an inner portion formed from the RIM polyurethane material 105, as shown in FIG. 10.
The steps of applying the mold release agent 102 and the in-mold coating material 103 have hitherto occupied so long a time in a molding cycle as to hinder any appreciable shortening of the molding cycle. The spraying of the in-mold coating material 103 has presented a problem, since it adheres also to Parting Line surfaces 107 contiguous to the cavity wall 101 and forms a burr 108 on the molded product 106, as shown in FIG. 10. A deburring job is, therefore, necessary.
The applicant, therefore, developed an invention covering a process for manufacturing a two-colored molded product of polyurethanes by RIM techniques which comprises injecting a small amount of a mixture of a polyurethane material, a mold release agent, a coloring agent and a catalyst into a mold cavity having a reduced pressure so that it may adhere to the whole surface of the wall of the cavity and form the surface portion of a molded product. Then another polyurethane material is injected into the cavity to form its inner portion (Japanese Patent Application Laid-open Specification No. 6-143340). This process has made it possible to eliminate the steps of applying the mold release agent and the in-mold coating material to the cavity wall before starting the molding step, and substantially overcomes the problem of deburring as stated above.
There has, however, been a problem facing the new process, since no polyurethane material that can be considered optimum has been found for use in forming the surface portion of a molded product. Light resistance and wear resistance are required of a surface-forming polyurethane material. It is preferable from the standpoint of light resistance to use a non-yellowing material made by employing an aliphatic isocyanate. The non-yellowing material, however, is usually so slow in its urethane reaction as hardly to cure in several minutes after adhering to the wall of the mold cavity. Therefore, it not only requires a greatly prolonged molding cycle, but is also likely to be driven away by the polyurethane material which is injected for forming the inner portion of a molded product, and its surface portion is likely to have an unduly reduced thickness particularly in the vicinity of the gate. Moreover, the non-yellowing material is usually low in wear resistance, too.